I lived about 3 blocks from a grocery in a downtown core for 10 years and rarely drove and walked it then. It was actually more of a hassle. My mom lived about 5 miles away. She'd pick me up on Sundays and we'd go shop. She'd cook dinner then drop me off at my apartment in that downtown core. This was a lot better.
Basically the car was great for the stuff you buy a bit more of and walking is great to pick up the stuff you want for this meal. Then again there were 5 restaurants on my walk from work to home so I just ate out all the time.
Most places are also not SF weather wise. This was in downtown St. Louis. It would be 85-90 the warm part of the year and sub 40 the rest of the year. Walking with heavy grocery bags in that weather or in winter requires changing clothes. Or pop in the car and get more moved and no sweat (literally). You go 5 blocks with a granny grocery cart in 95 weather and you'll need a shower when you get home.